The transmission of various types of digital data between computers continues to grow in importance. The predominant method of transmitting such digital data includes coding the digital data into a low frequency base data signal and modulating the base data signal onto a high frequency carrier signal. The high frequency carrier signal is then transmitted across a network cable medium, via RF signal, modulated illumination, or other network medium, to a remote network node.
At the remote computing station, the high frequency carrier signal must be received and demodulated to recover the original base data signal. In the absence of any distortion of the carrier signal across the network medium, the received carrier would be identical in phase, amplitude, and frequency to the transmitted carrier and could be demodulated using known mixing techniques to recover the base data signal. The base data signal could then be recovered into digital data using known sampling algorithms.
One problem with such networks is that the network topology tends to distort the high frequency carrier signal due to numerous branch connections and different lengths of such branches causing numerous reflections of the transmitted carrier. The high frequency carrier is further distorted by spurious noise caused by electrical devices operating in close proximity to the cable medium. Such problems are even more apparent in a network which uses home telephone wiring cables as the network cable medium because the numerous branches and connections are typically designed for transmission of plain old telephone system (POTS) signals in the 3–10 kilohertz frequency and are not designed for transmission of high frequency carrier signals on the order of 7 Megahertz. Further yet, the high frequency carrier signal is further distorted by turn-on transients due to on-hook and off-hook noise pulses of the POTS utilizing the network cables.
Such distortion of frequency, amplitude, and phase of the high frequency carrier signal degrades network performance and tends to impede the design of higher data rate networks. Known techniques for compensating for such distortion and improving the data rate of a network include complex modulation schemes.
Utilizing a complex modulation scheme such as quadrature amplitude modulation (QAM) data, both the amplitude and phase of the high frequency carrier are modulated to represent I and Q components of a base data signal. Referring to FIG. 1, a 4-QAM modulation constellation 10 is shown. In operation, each data symbol is represented by an I-value of +1 or −1 and a Q-value of +1 or −1 such that the data symbol can be represented by one of the four modulation states 12(a)–(d) in constellation 10. Each constellation state 12(a)–12(d) represents a unique combination of carrier amplitude and phase. For example, constellation state 12(a) represents a carrier amplitude of 14 and a carrier phase 16.
A complex modulation transmitter typically uses a look up table to generate an I-channel and a Q-channel baud rate data signal. An upsampler then inserts additional sample values of zero to increase the input sample frequency to a frequency greater than the desired carrier frequency. A complex mixer then mixes each of the I-channel signal and the Q-channel signal by digital sine waves and digital cosine waves of the carrier frequency as appropriate to generate a modulated carrier signal. Narrow band digital filters are then used to remove harmonics and to assure that the transmitted signal has a strong signal to noise ratio within the desired band without excessive noise in the side bands.
A problem with such systems is that a carrier frequency on the order of 7 MHz is typically represented by digital values clocked at a frequency on the order of 32 MHz. As such, a digital signal processor (DSP) implementation of a QAM (or I/Q) transmitter can consume many gates or may not even be possible to implement in a high speed DSP without architectural innovation. What is needed is a device and method for I/Q modulation, upsampling, and digital filtering that does not suffer the disadvantages of known systems.